(1) Field of the Invention
The present invention relates to stabilized, liquid diepisulfide based prepolymers, a method for preparing such prepolymers, a method for polymerizing curing the prepolymers into final polymer materials and their lo application for making transparent optical articles such as ophthalmic lenses.
(2) Description of the Prior Art
Diepisulfide based resins are of major interest in the optical field owing to their high refractive indexes of 1.7 or more.
Document U.S. Pat. No. 5,807,975 discloses thermal polymerizaton and curing of diepisulfides, in particular for the manufacture of optical materials such as a lens material for spectacles. This thermal polymenzation/curing of diepisulfides lasts for several hours, generally about 20 hours.
Document EP-0.942.027 discloses an optical resin prepared by curing a polymerizable composition comprising a (thio)epoxy compound having at least one intramolecular disulfide bond The cuing catalysts which may be used arc typically tertiary amines, phosphines, Lewis acids, radical polymerization catalysts and cationic polymerization catalysts. Lengthy polymerization time are reported
Document WO 00/26272 discloses a process for making thio containing resins which comprises polymerizing a composition comprising at least one diepisulfide compound and as polymerization catalyst, an alkaline metal, alkaline-earth metal or ammonium salt of an acid having a pKa ranging from 0.5 to 14.
Although, such a polymerization process leads to fast polymerization/curing of the composition, die process is very difficult to control due to the highly exothermic polymerization reaction and often results in a charred resin inappropriate for making optical articles such as ophthalmic lenses.
It has now been discovered that it is possible to safely prepare a liquid diepisulfide based prepolymer which is stable in time, the prepolymer being able to be later safely polymerized/cured into a transparent resin useful for making optical articles such as ophthalmic lenses.
Thus, an object of the present invention is a stabilized, liquid, diepisulfide based prepolymer useful for making transparent final resins.
It is another object of the present invention to provide a process for is making a stabilized, liquid, diepisulfide based prepolymer.
It is a further object of the present invention to provide a process for making a transparent optical article through fast polymerization/curing of a stabilized, liquid, diepisulfide based prepolymer.
These and other objects arc attained by providing a stabilized, liquid, diepisulfide base prepolymer having a viscosity at 25xc2x0 C. ranging from 20 to 33000 cPs, preferably from 100 to 1000 cPs.
By a xe2x80x9cstabilizedxe2x80x9d prepolymer there is meant a prepolymer the viscosity of which does not vary and particularly does not increase more than 20% when stored at 20xc2x0 C. over a period of 7 days, preferably 10 days and more preferably one month.
By a xe2x80x9cliquidxe2x80x9d prepolymer there is meant a prepolymer which can be poured and cast into a casting mold such as a casting mold for making ophthalmic lenses.